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Simple terminology question:

  • Is there a hard boundary between in vitro and ex vivo?
  • Is there a hard boundary between in vivo and ex vivo?

Suppose a sensory neuron is electrically recorded in the following settings:

  1. Surgically accessing through the animal's skin, and patch-clamped,
  2. Tissue surrounding the neuron is almost separated, but still dangling from the larger part of the animal, and patch-clamped,
  3. Same as above, but the slight dangling connection is cut,
  4. The sensory neuron is isolated and put in an artificially oxygenated medium

The sensory neuron is assumed to have purely feedforward signal, and is perfectly healthy in all conditions. Which conditions, if any, are in vivo, in vitro, or ex vivo? (Feel free to propose better hypothetical experiments. I know this example is not the best.)

Am I the only one confused about these terminologies?

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up vote 3 down vote accepted

Ex vivo simply means "outside the normal, living organism", whereas in vitro means "within glass", usually in a cultured system. They are not exactly same as there is no need for the work to be done in a culture system, although both are not in vivo. Harvested tissue could be examined ex vivo without being in a test tube, or surgical systems could be tested ex vivo on artificial organ models. In vivo requires the natural, living organism, although it depends on just how much you are perturbing the natural.

To answer your scenarios,

  1. in vivo
  2. ex vivo, as it is not within the normal organism, although I could understand someone taking issue with that; definitely not in vivo, which is usually for normal conditions only, even including temperature.
  3. ex vivo
  4. If you mean the artificially oxygenated environment of a tissue culture system, then in vitro; if you mean the artificially oxygenated environment of a lab bench with an open $O_2$ valve, then ex vivo would suffice.
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(Not only) Wikipedia states that in vitro and in vivo have a very hard boundary between them. I something is studied in the context of an organism, without being extracted, it is in vivo, as soon as it is extracted it is in vitro.

Strictly speaking, I would count only the first of your neuron examples as in vivo, because it is still expected to be working like it does inside the organism. As soon as you start having it dangling out, the original "environment" is destroyed.

The relation between ex vivo and in vitro is less defined. I would count example 2 as ex vivo, but not in vitro, as the Wikipedia article for ex vivo describes. Same goes for example 3.

Example 4 is of course in vitro. Debatable is, if in vitro excludes ex vivo. It is still taken out of an organism, not grown artificially, so I would still consider it ex vivo.

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